Our immediate goals are: 1) To determine the molecular basis for the decreases in hepatic mitochondrial content and cytochrome oxidase activity in aged Fisher 344 rats. 2) To use computer-assisted techniques to evaluate mitochondrial three-dimensional structure as a function of aging and to develop mathematical expressions to describe mitochondrial complexity. Our primary model will be Fisher 344 rats from the aging colony of NIA. For comparative purposes, other animals will be evaluated i.e. the Sprague-Dawley rat (continues to grow beyond sexual maturity), and partially hepatectomized rats (to compare mitochondria from newly synthesized liver cells in old animals with those of old liver cells.